1. Field of the Invention
This invention relates to easily-sinterable alumina-titania composite powder for use as starting materials of aluminum titanate ceramics which have a high mechanical strength and a low thermal coefficient of expansion.
2. Prior Arts
Aluminum titanate (Al.sub.2 TiO.sub.5) has been long known as a ceramic of low thermal expansivity. This low thermal expansivity is said to be afforded by cracks caused in particles due to significant anisotropy in thermal expansivity of aluminum titanate crystals. These cracks, however, also act adversely to reduce the strength of aluminum titanate, restricting the application use thereof.
Conventional processes for preparing aluminum titanate are generally classified, by form of starting materials to be employed, into three: (i) a process in which powders of alumina (Al.sub.2 O.sub.3) and titania (TiO.sub.2) are used as starting materials in the form of a mixture; (ii) a process in which aluminum titanate synthesized is used after breaking; and (iii) a process in which composite powders of Al.sub.2 O.sub.3 -TiO.sub.2 prepared by oxidation of halides of aluminum and titanium in a gaseous phase are employed as starting materials.
Sintered bodies prepared from the powder mixture as specified in (i) above are low in density, although it may be improved to some extent by selecting Al.sub.2 O.sub.3 and TiO.sub.2 feedstocks. The sintered bodies are also poor in strength because they show domain structure in which Al.sub.2 TiO.sub.5 crystals grow, causing large cracks therein. It has been reported that the domain can be reduced and the bending strength can be improved by carrying out the sintering at a high rate or by preliminarily adding a small amount of aluminum titanate to the mixture. However, the effect is not so noticeable and the attained highest strength is still as low as 2 Kg/mm.sup.2 (Kenya Hamano et al., "Yogyo-kyokai-shi," 91 (2) 94-101 (1983)), which is not satisfactory.
In the case that the aluminum titanate powders as specified in (ii) above are employed, it has been known that the obtained sintered bodies have no domain structure and that the particle diameters of the sintered bodies are reduced and the bending strengths thereof are improved if Al.sub.2 O.sub.3 particles chipped from Al.sub.2 O.sub.3 milling balls are allowed to be mixed into the materials during a long ball-milling operation by the ball mill. For example, when the materials are sintered at a low temperature of 1300.degree. C. for a long time of 8 hours, the obtained sintered bodies have a high bending strength of 8.6 Kg/mm.sup.2. However, the coefficient of thermal expansion is as high as about 0.4% between room temperature and 1000.degree. C. In addition, the density of the sintered bodies is as low as 82% of the theoretical density. In contrast, when the materials are sintered at a high temperature of 1500.degree. C. for about 4 hours, the coefficient of thermal expansion is as low as 0.1% between room temperature and 1000.degree. C. and the density is relatively high, 94% of the theoretical density. However, the strength is not satisfactory. The bending strenth is 5 to 6 Kg/mm.sup.2 at highest (Yutaka Ohya et al., "Yogyo-kyokai-shi," 91 (6) 289-97(1983)).
There is a report that relatively high strength can be attained when a third component such as MgO, SiO.sub.2 and ZrO.sub.2 is added to the powders of (i) or (ii) above (Kenya Hamano et al., "Nippon Kagakukai-shi," (10) 1647-55(1981)). In this case, however, the strength is still unsatisfactory. The attainable highest bending strength is 5 to 6 Kg/mm.sup.2.
The above-mentioned process (iii) for/preparing Al.sub.2 O.sub.3 -TiO.sub.2 composite powder through oxidation of aluminum and titanium halides in a vapour phase includes an r.f. plasma method proposed by Gani et al. and a method using a tubular reactor proposed by Tokunaga et al. The composite powder obtained by Gani et al. have an average particle diameter of 30 to 170 nm and crystalline forms such as .delta.-Al.sub.2 O.sub.3 within a range in which Al.sub.2 O.sub.3 is major and rutile-TiO.sub.2 within a range in which TiO.sub.2 is major, and intermediate phases such as .beta.-Al.sub.2 O.sub.5 in the case where the molar ratio of Al.sub.2 O.sub.3 to TiO.sub.2 is around 1:1 and X-phase together with .delta.-Al.sub.2 O.sub.3 in the case where the molar ratio is around 3:1 (Gani et al., Journal of Materials Science, Vol. 15, 1915-1925 (1980)).
In the composite powders of Tokunaga et al., Al.sub.2 O.sub.3 is amorphous and TiO.sub.2 is in the form of anatase when Al.sub.2 O.sub.3 is little and rutile when Al.sub.2 O.sub.3 is in a considerable amount. Under the conditions in which the reaction temperature is 1000.degree. C. or higher and the molar % of Al.sub.2 O.sub.3 is 20 or higher, -Al.sub.2 TiO.sub.5 is detected (Tokunaga et al., "Nippon Kagakukaishi," (11) 1758-1762 (1982)).
According to the plasma method, the particles are formed by crystallization after they have been once molten, taking a course of a vapor phase.fwdarw.a liquid phase.fwdarw.a solid phase. Whereas, according to the method using the tubular reactor, crystallization occurs directly from the vapor phase. Although these two methods differ in produced crystalline phases due to the difference in reaction temperatures and crystallization mechanisms (through or not through a liquid phase), either of Gani et al. or Tokunaga et al. produce aluminum titanate (Al.sub.2 TiO.sub.5) as a major crystalline phase around the molar ratio of 1:1.
When such Al.sub.2 O.sub.3 -TiO.sub.2 powder is used, they can not afford sufficient strength to the sintered bodies.